This application claims the priority of Korean Patent Application No. 2003-57275, filed on Aug. 19, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an electrolyte composition, a lithium battery using the same, and a method of manufacturing the lithium battery, and more particularly, to an electrolyte composition with improved stability in the case of overcharging, a lithium battery using the electrolyte composition, and a method of manufacturing the lithium battery.
2. Description of the Related Art
As an increasing number of electronic devices, especially, portable devices such as personal digital assistants (PDAs), mobile phones, notebook computers, etc. spreads widely and become used in more application fields, there has been intensive research on batteries as driving sources for such devices with the need for smaller, thin design, lightweight, high-performance batteries.
Among various kinds of batteries lithium batteries have been used as typical driving power sources for portable devices due to the lightweight and high energy density thereof. A lithium battery consists of a cathode, an anode, a separator, and an electrolyte solution interposed between the cathode and the anode to provide a path of lithium ions. The lithium battery produces electrical energy by oxidation/reduction reactions occurring when the intercalation/deintercalation of lithium ions occurs at the cathode and the anode.
However, when the lithium battery is overcharged due to a malfunction of a charger, thereby causing a rapid rise in voltage, a large amount of lithium ions may be separated at the cathode while the intercalation of a large amount of lithium ions occurs at the anode. This thermal instability between the cathode and the anode leads to decomposition of an organic solvent contained in the electrolyte solution, rapid generation of heat, and eventually a thermal runaway phenomenon, thereby raising safety concerns.
To solve this problem, there has been tried to suppress the overcharging of the lithium battery by changing the composition of the electrolyte solution or by adding an additive to the electrolyte composition. For example, U.S. Pat. No. 5,879,834 discloses a method of enhancing the safety of a battery by using a biphenyl substance as an additive that can be polymerized at a voltage when the battery is overcharged and thus raise internal resistance. Japanese Patent Laid-open No. hei 9-17447 discloses a benzene compound that can reversibly oxide and reduce at a potential that is higher than a positive electrode potential when overcharging occurs. Japanese Patent Laid-open No. hei 5-36439 discloses the use of an electrolyte solution containing at least one of a chain carbonic ester, a cyclic carbonic ester, polypropylene carbonate (PC), and alkyl benzene in a battery system with a current cutoff device for preventing an interval voltage increase in the battery. Japanese Patent Laid-open No. hei 9-106835 discloses a non-aqueous, rechargeable lithium battery using a polymeric aromatic additive such as biphenyl, 3-chloro thiophene, franc, etc. as an additive for the electrolyte solution and a method of improving the safety of the battery. Japanese Patent Laid-open No. 2001-15155 discloses the use of an alkyl benzene derivative with a tertiary carbon near a phenyl group or cyclohexylbenzene. Hydrogen bound to the tertiary carbon is unstable and thus is easily decomposed into hydrogen gas at a high voltage, thereby initiating the operation of a current cutoff device and thus suppressing overcharging.
However, the biphenyl compound leads to accumulation of a conductive polymer inside the electrodes and on the separator when the battery is charged and discharged and an increase in the thickness of the battery, thereby causing internal shorting out during the charging/discharging cycles.
In the case of using the benzene compound, which has to be simultaneously oxidized and reduced at the cathode and the anode, a large amount of additive is required to prevent overcharging of the battery. However, the cycle lifetime or high-temperature storage characteristic of the battery deteriorates due to side reactions.
In the case of using the alkyl benzene compound to prevent overcharging, where the current cutoff device is operated by hydrogen gas produced as a reaction product, a more complicated battery system is required because overcharging cannot be prevented using the alkyl benzene compound alone.
Japanese Patent Laid-open No. 2001-15155 discloses that the amount of an additive preferably ranges from 1-5% by weight. In a polymer battery using a polymer electrolyte in which mobility of the additive is lower than in an electrolyte solution of a lithium ion battery, the amount of the additive has to be increased. However, when the amount of the additive, for example, cyclohexylbenzene, is increased above 5% by weight, the overcharging characteristic, lifespan characteristic, and swelling characteristic of the battery deteriorate.